In the present work, we screened various microbial metabolites in search of HIV-1 and HTLV-I protease inhibitors and found strong inhibitory activity in the culture medium of a soil isolate, Streptomyces sp. AL-322. Four kinds of retrovirus protease inhibitors (RPI-856 A, B, C and D) were isolated as white powder from the culture filtrate of this strain by column chromatography using Diaion HP-20, Sephadex LH-20, ODS reversed phase HPLC and SP-2SW ion exchange HPLC. The structures of these inhibitors were elucidated by physico-chemical properties, chemical reactions and spectral analyses, as valyl-ADPAA-leucyl-AOPBA-valyl-valyl-aspartic acid (RPI-856 A and B) and valyl-ADPAA-leucyl-AOPBA-valyl-valine (RPI-856 C and D) [ADPAA = 2 amino-2-(3,5-dihydroxyphenyl)acetic acid, AOPBA= 3-amino-2-oxo-4-phenylbutyric acid]. RPI-856 A and B, and RPI-856 C and D were both determined to be diastereomers each other on the asymmetric carbon in AOPBA. These four inhibitors strongly inhibited in vitro HIV-1 protease and HTLV-I protease both derived from recombinant Escherichia coli with IC50 of 10^-7 ~ 10^-8 M. This paper describes the taxonomy of the producing strain, the purification, the structure and protease inhibitory activities of RPI-856 A, B, C and D.